Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Thu Dec 21 2023 Valid 12Z Sun Dec 24 2023 - 12Z Thu Dec 28 2023 ...Heavy Snow Threat Spreads to the North-Central Plains Christmas Eve... ...Heavy Rain Threat from the Lower Mississippi Valley to the Southeast Christmas Eve and Christmas and then the Mid-Atlantic toward Midweek... ...Overview... A closed southern stream upper low off southern California will open and eject northeast to the Colorado Rockies this weekend before phasing with a digging northern stream trough over Colorado to become a multi-phase and slow moving low that shifts slowly downstream through late next week. The weekend translation of the dynamic system will spawn downstream south-central Plains cyclogenesis/frontogenesis with phasing/occlusions keeping eastward motion slow through Monday/Christmas. Return Gulf of Mexico moisture and instability will fuel an emerging area of enhanced rains and convection over the eastern portions of the Southern Plains/Lower MS Valley then Southeast Sunday though Monday/Christmas that then shifts main focus to the Mid-Atlantic and onward through midweek. Meanwhile, the threat of heavy snow is expected to shift from the southern Rockies to the north-central Plains Saturday night into Monday/Christmas with the lingering/re-phasing low causing more snow concerns on the back side over the central Plains to Upper Midwest into midweek. ...Guidance/Predictability Assessment... Plenty of uncertainty during the medium range period. A general model blend consisting of the 00z EC/CMC/UKMET and 06z GFS were used through day 5. The last few runs of the GFS have been too fast with the evolution of the upper trough/low through the Plains, Mississippi Valley and Midwest through the period compared to the rest of the guidance, but especially on day 3. It's for that reason that it was weighted less in the day 3 blend. The Canadian and UKMET diverge a bit from the rest of the guidance with respect to a potential shortwave propagating through the Northwest on day 4 so they had their weighting reduced a bit for that. The ensembles were introduced on day 5 and weighted heavily through the remainder of the period to account for a lot of dispersion in the guidance. ...Weather/Hazards Highlights... Complex upper trough progression and Plains surface system organization supports a heavy snow threat from the southern Rockies through the north-central Plains in time for Christmas Eve and Christmas. The WPC Winter Weather Outlook (WWO) shows rising/enhanced plowable commahead snow probabilities. System/energy downstream propagation will generate an emerging wet pattern with potential for strong convection set to erupt Christmas Eve with focus from eastern portions of the southern Plains through the Lower Mississippi Valley and central Gulf Coast. Increasing instability with upwards of a 50kt LLJ and 2 SD above normal PWs will combine with favorable upper difluence/right entrance region upper jet dynamics and also from a strong/surging subtropical jet. Maintained the Day 4/Christmas Eve Slight Risk ERO. Expect a heavy rainfall signature from the Southeast/Southern Appalachians to the Mid-Atlantic Christmas into Tuesday given system translation/re-phasing and added Atlantic moisture. The northward expansion of the broad precipitation shield with frontal wave propagation is also likely to meanwhile spread moderate precipitation amounts up from the Midwest through the Northeast with steady erosion of a leading/cooled eastern U.S. surface ridge. In this period, this should equate to some northern tier wintry precipitation from the Central Plains to Great Lakes to northern New England, though uncertainty remains given the varied timing of the deterministic solutions. Meanwhile well upstream, mean upper trough amplitude set to persist over the eastern Pacific should prove slow to translate toward the West Coast, but system intensities and gradually closer proximity suggests that lead flow impulses and deep layered/long fetch southerly moisture flow should allow for some periods of moderate to heavy precipitation into mainly coastal areas and especially south facing favored terrain from the Pacific Northwest to northern California Sunday night through Tuesday, with additional activity into Wednesday/next Thursday. A WPC ERO Marginal Risk area was introduced for Day5/Christmas when guidance shows the best defined atmospheric river signal into coastal Washington and in particular the Olympics. Temperatures should trend quite warm across the central U.S., especially into the Northern Plains and Upper Midwest this weekend. Daytime highs could be 20 to 25 degrees above normal in this region, while lows of 25-35 degrees above average are expected. A few record highs are forecast while record warm minimum temperatures may be widespread. Elsewhere, daytime high anomalies return back to normal across the East as upper ridging builds and upper troughing into the West should result in a cooling trend by the weekend. Schichtel Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw